chemical reactions. chemical equation describes what you had before and then after the change...
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Chemical Reactions
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Chemical EquationDescribes what you had before
and then after the changeReactants: substances that
undergo changeProducts: new substances
formed
Reactants Products
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Chemical EquationChemical equation:
representation of a chemical reaction in which the reactants and products are expressed as formulas
Examples:
C + O2 CO2
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Conservation of MassAntoine Lavoisier 1743-1794
◦Law of conservation of mass: mass is neither created nor destroyed in a chemical reaction
Mass of the products is always equal to the mass of the reactants
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Balancing Equations
H2 + O2 H2O
NOT BALANCED!
Number of atoms on the reactant side MUST equal the number of atoms on the product side!!!
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Balancing Equations
H2 + O2 H2O
Use Coefficients to balance equations
2H2 + O2 2H2O
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ExampleCu + O2 CuO
Balanced or unbalanced???
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ExamplesNa + Cl2 NaCl
H2 + Cl2 HCl
AgNO3 + Cu Cu(NO3)2 + Ag
BaS + PtF2 BaF2 + PtS
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Types of Reactions
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Classifying reactionsReactions are often classified by
the type of reactant or the number of reactants and products
General types include:◦Synthesis reaction◦Decomposition reaction◦Single-replacement◦Double-replacement◦Combustion
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SynthesisTwo or more substances react to
form a single substance
A + B AB
Example: 2Cu + O2 2CuO
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DecompositionCompound breaks down into two
or more simpler substances
AB A + B
Example:CaCO3 CaO +
CO2
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Single ReplacementOne element takes the place of
another element in a compound
A + BC B + AC
Example:Cu + 2AgNO3 2Ag +
Cu(NO3)2
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Double ReplacementTwo different compounds exchange
positive ions and form two new compounds
AB + CD AD + CB
Example:CaCO3 + 2HCl CaCl2 + H2CO3
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CombustionSubstance reacts rapidly with
oxygen, often producing heat and light
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Breaking and Forming BondsBreaking bonds:
◦Requires energy Example: Propane grills have a starter
that produces a spark
Forming bonds:◦Releases energy
Example: heat and light are given off by a propane grill
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Exothermic ReactionChemical reaction that releases
energy to its surroundings
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Endothermic ReactionChemical reaction that absorbs
energy from its surroundings
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Exothermic vs EndothermicExo:
◦Reactants have more energy than products AB+ CD+ 450 J AD + CB
Endo: ◦Products have more energy than
reactants AB + CD AD + CB + 450 J
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Reaction Rates
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Reactions over timeAny change that happens over a
period of time can be expressed as a rate
Reaction rate: rate at which reactants change into products over time◦Tell us how fast a reaction is going
How fast reactants are being consumed How fast the products are being formed How fast energy is being absorbed or
released
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Factors Affecting Reaction RatesReaction rate depends on how
often particles collide◦More frequent = increase reaction rate◦Less frequent = decrease reaction rate
TemperatureSurface areaConcentrationStirringCatalysts
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TemperatureIncrease in temperature =
increase in reaction rateDecrease in temperature =
decrease in reaction rate
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Surface AreaAn increase in surface area
increases the exposure of reactants to one another
An increase in surface area = increase in reaction rate
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StirringStirring causes collisions to occur
more rapidlyStirring = increase in reaction
rate
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ConcentrationConcentration = number of
particles in a given volumeGreater the concentration =
increase in reaction rate
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CatalystsCatalyst: substance that affects
the reaction rate without being used up in the reaction◦Used to speed up reactions or used
to enable a reaction at a lower temperature
◦Written over the arrow because it is not used up
2SO2 + O2 2SO3
V2O3
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CatalystsIn order for a reaction to occur,
particles must collide with enough energy to break the bond◦Catalyst lowers this energy barrier◦Provides a surface area for reacting
particles to come together